Oxygen is vital to most living things on Earth, and the way to get it is through breathing. Most of the creatures living on land use lungs to breathe, such as mammals, reptiles, etc.

There are also some terrestrial animals that do not have lungs as respiratory organs. For example, locusts breathe through trachea; earthworms breathe through moist body walls.

Oxygen on land exists in the air, accounting for about 21% of the air content. Water also contains a certain amount of oxygen, commonly known as dissolved oxygen. How do animals living in the ocean breathe to obtain dissolved oxygen in the water?

1 Breathe with gills

Most of the animals living in water use gills to breathe, such as fish, shrimp, crab, shellfish and so on. The feathery gills are covered with capillaries, and the walls of these capillaries are thin, allowing gas to penetrate easily.

2 Breathe with your "ass"

Yes, you read that right, some marine animals do use their "butts" to breathe, such as sea cucumbers that everyone loves to eat.

3 Breathe with "skin"

Most coelenterates breathe through "skin", where the term "skin" refers to the body wall of the coelenterates. Common coelenterates include jellyfish and sea anemones.

Coelenterates do not have respiratory organs and rely on body wall cells for gas exchange with the surrounding water. The body structure of jellyfish is very simple.

Unlike jellyfish, which breathe directly through their "skin," starfish breathe through dermal gills (the starfish's breathing organs) that grow on their bodies.

The skin gills have only epidermis, and their inner cavity is directly connected with the body cavity, which can directly diffuse the collected oxygen into the body cavity of the starfish.

4 Breathe with your lungs

Marine mammals and marine reptiles use lungs to breathe. Breathing with lungs is inseparable from air, so marine animals that breathe with lungs need to regularly return to the surface to breathe air.

Have you ever seen a whale spouting a jet of water on the ocean? That's the whale breathing.

Starfish have eyes—one at the end of each tentacle, but the purpose of these eyes has not been understood.

For a long time, starfish were thought to be simple animals. Because their eyes are also relatively simple, and they have no brains, it is difficult to understand how they see, or even whether they can see.

A 2014 study showed that tropical starfish's eyes can form blurry images, preventing them from moving too far from home. Scientists tend to use "sea star" as the name for a starfish, rather than "starfish," because invertebrates aren't really fish.